On 8/9/21 11:39 PM, Florian Westphal wrote:
Pavel Skripkin <paskripkin@xxxxxxxxx> wrote:
I think, there a missing lock in this function:
for_each_possible_cpu(cpu) {
ct = per_cpu(nft_ct_pcpu_template, cpu);
(*)
if (!ct) >> break;
nf_ct_put(ct);
per_cpu(nft_ct_pcpu_template, cpu) = NULL;
}
Syzbot hit a UAF in nft_ct_tmpl_put_pcpu() (*), but freed template should be
NULL.
So I suspect following scenario:
CPU0: CPU1:
= per_cpu()
= per_cpu()
nf_ct_put
per_cpu = NULL
nf_ct_put()
* UAF *
Hi, Florian!
Yes and no. The above is fine since pcpu will return different pointers
for cpu 0 and 1.
Dumb question: why per_cpu() will return 2 different pointers for CPU 1
and CPU 0? As I understand for_each_possible_cpu() will iterate over all
CPUs which could ever be enabled. So, we can hit situation when 2
concurrent processes call per_cpu() with same cpu value (*).
The race is between two different net namespaces that race when
changing nft_ct_pcpu_template_refcnt.
This happens since
commit f102d66b335a417d4848da9441f585695a838934
netfilter: nf_tables: use dedicated mutex to guard transactions
Before this, all transactions were serialized by a global mutex,
now we only serialize transactions in the same netns.
Its probably best to add
DEFINE_MUTEX(nft_ct_pcpu_mutex) and then acquire that when we need to
inc/dec the nft_ct_pcpu_template_refcnt so we can't have two distinct
cpus hitting a zero refcount.
Would you send a patch for this?
Anyway, I think, moving locking a bit higher is good here, let's test
it. I will prepare a patch, if it will pass syzbot testing, thanks!
#syz test
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
With regards,
Pavel Skripkin
>From 616e99fd3ac738b2b5e43c5bc57f6f8cc7a49da0 Mon Sep 17 00:00:00 2001
From: Pavel Skripkin <paskripkin@xxxxxxxxx>
Date: Tue, 10 Aug 2021 00:13:38 +0300
Subject: [PATCH] netfiler: protect nft_ct_pcpu_template_refcnt with mutex
/* .... */
Signed-off-by: Pavel Skripkin <paskripkin@xxxxxxxxx>
---
net/netfilter/nft_ct.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/net/netfilter/nft_ct.c b/net/netfilter/nft_ct.c
index 337e22d8b40b..99b1de14ff7e 100644
--- a/net/netfilter/nft_ct.c
+++ b/net/netfilter/nft_ct.c
@@ -41,6 +41,7 @@ struct nft_ct_helper_obj {
#ifdef CONFIG_NF_CONNTRACK_ZONES
static DEFINE_PER_CPU(struct nf_conn *, nft_ct_pcpu_template);
static unsigned int nft_ct_pcpu_template_refcnt __read_mostly;
+static DEFINE_MUTEX(nft_ct_pcpu_mutex);
#endif
static u64 nft_ct_get_eval_counter(const struct nf_conn_counter *c,
@@ -525,8 +526,10 @@ static void __nft_ct_set_destroy(const struct nft_ctx *ctx, struct nft_ct *priv)
#endif
#ifdef CONFIG_NF_CONNTRACK_ZONES
case NFT_CT_ZONE:
+ mutex_lock(&nft_ct_pcpu_mutex);
if (--nft_ct_pcpu_template_refcnt == 0)
nft_ct_tmpl_put_pcpu();
+ mutex_unlock(&nft_ct_pcpu_mutex);
break;
#endif
default:
@@ -564,9 +567,13 @@ static int nft_ct_set_init(const struct nft_ctx *ctx,
#endif
#ifdef CONFIG_NF_CONNTRACK_ZONES
case NFT_CT_ZONE:
- if (!nft_ct_tmpl_alloc_pcpu())
+ mutex_lock(&nft_ct_pcpu_mutex);
+ if (!nft_ct_tmpl_alloc_pcpu()) {
+ mutex_unlock(&nft_ct_pcpu_mutex);
return -ENOMEM;
+ }
nft_ct_pcpu_template_refcnt++;
+ mutex_unlock(&nft_ct_pcpu_mutex);
len = sizeof(u16);
break;
#endif
--
2.32.0
